OSKALOOSA, Iowa — The slugfest between Republican presidential frontrunners Mitt Romney and Newt Gingrich is escalating, with the two sparring from a distance over attack ads that have come to dominate the volatile contest.

The rift underscores the contrasting campaign styles of the two men as they ready their final pitches to voters in Iowa and New Hampshire. And it left each decrying a new campaign finance system — fueled by deep-pocketed political action committees — that each benefits from.

In Iowa, Gingrich vowed his White House bid would remain positive, while in the next breath he labeled the Romney camp's tactics "disgusting." The former House speaker, known for a bare-knuckles campaign style when he engineered the GOP takeover of the House in the 1990s, suggested at a campaign stop on Tuesday that his opponents "hire consultants who get drunk, sit around and write stupid ads."

Romney protested that he couldn't control the independent PAC expenditures, but pointedly declined to disavow the ads. The former businessman and Massachusetts governor, who's been on the stump in New Hampshire, is seeking to project a tough, pragmatic image, allowing there's "no whining in politics."

"I'm a big boy," he said on MSNBC.

Meanwhile, Ron Paul has emerged as a wild card in the race. The blunt-spoken Texas congressman is set to hit the trail in Iowa again on Wednesday, along with other candidates in the field. With less than two weeks to go until the first-in-the nation Iowa caucuses, the race remains unpredictable, as voters weigh electability against conservative credentials.

Jenny Turner, a 31-year-old wedding videographer from Mount Pleasant, Iowa, summed it up this way: "My heart is with Newt. But Mitt Romney is the back of my mind."

The bickering over negative ads has highlighted the role of so-called super PACs, independent groups which may accept unlimited donations but are not supposed to directly coordinate with candidates. Such groups have sprung up to work on behalf of every serious Republican candidate in the wake of a U.S. Supreme Court decision last year that allowed people, unions and corporations to donate unlimited sums of money to outfits advocating the election or defeat of candidates.

Romney has labeled the system a "disaster"; Gingrich calls it "a nightmare."

Still, each has PACs leveraging dollars on their behalf.

Among the most visible is the pro-Romney PAC Restore our Future, run by former Romney aides. It has been blanketing the airwaves in Iowa with a series of caustic ads painting Gingrich as an ethically-challenged Washington power broker. The onslaught has chipped away at Gingrich's poll numbers in the state

Gingrich has challenged Romney to demand the ads come down.

However, two pro-Gingrich PACs have also been raking in cash. Gingrich's longtime aide Rick Tyler just signed on with one of them.

Gingrich trails Romney badly in fundraising and his campaign had been deeply in debt following staff upheaval earlier this year. The PAC spending could help him rapidly make up that gap. Gingrich said Wednesday he would disavow any PAC that runs negative ads, but that doesn't mean they could not do so anyway.

Gingrich and Romney were each expected back on the campaign trail Wednesday, one of the last full days before the campaigns go dark for the holidays. Romney had a full day of campaign stops in New Hampshire, a must-win state for him. He was set to pick up the endorsement of a key conservative activist.

Gingrich was collecting the support from state House speakers in Iowa and New Hampshire and was set to hold events in both states. He was then set to rush to Virginia, which the former Georgia congressman now calls home, to help ensure he has the needed signatures to get on the ballot in the state.